Nr.6042-Disinfection with chlorine and chlorine dioxide

http://water.me.vccs.edu/courses/ENV110/lesson7.htm

What is Disinfection?

Before water treatment became common, waterborne diseases could spread quickly through a population, killing or harming hundreds of people.  The table below shows some common, water-transmitted diseases as well as the organisms (pathogens) which cause each disease.  More information on water-borne pathogens can be found in ENV 108. 

The primary goal of water treatment is to ensure that the water is safe to drink and does not contain any disease-causing microorganisms.  The best way to ensure pathogen-free drinking water is to make sure that the pathogens never enter the water in the first place.  However, this may be a difficult matter in a surface water supply which is fed by a large watershed.  Most treatments plants choose to remove or kill pathogens in water rather than to ensure that the entire watershed is free of pathogens.

Pathogens can be removed from water through physical or chemical processes.  You may remember that some previously discussed treatment processes, notably sedimentation and filtration, can remove a large percentage of bacteria and other microorganisms from the water by physical means.  Storage can also kill a portion of the disease-causing bacteria in water. 

Prechlorination and Postchlorination

Like several other water treatment processes, chlorination can be used as a pretreatment process (prechlorination) or as part of the primary treatment of water (postchlorination).  Treatment usually involves either postchlorination only or a combination of prechlorination and postchlorination. 

Prechlorination is the act of adding chlorine to the raw water.  The residual chlorine is useful in several stages of the treatment process - aiding in coagulation, controlling algae problems in basins, reducing odor problems, and controlling mudball formation.  In addition, the chlorine has a much longer contact time when added at the beginning of the treatment process, so prechlorination increases safety in disinfecting heavily contaminated water. 

Postchlorination is the application of chlorine after water has been treated but before the water reaches the distribution system.  At this stage, chlorination is meant to kill pathogens and to provide a chlorine residual in the distribution system.  Postchlorination is nearly always part of the treatment process, either used in combination with prechlorination or used as the sole disinfection process.  

Until the middle of the 1970s, water treatment plants typically used both prechlorination and postchlorination.  However, the longer contact time provided by prechlorination allows the chlorine to react with the organics in the water and produce carcinogenic substances known as trihalomethanes.  As a result of concerns over trihalomethanes, prechlorination has become much less common in the United States.  Currently, prechlorination is only used in plants where trihalomethane formation is not a problem.

 Chlorination Chemistry 
Introduction

When chlorine is added to water, a variety of chemical processes take place.  The chlorine reacts with compounds in the water and with the water itself.  Some of the results of these reactions (known as the chlorine residual) are able to kill microorganisms in the water.  In the following sections, we will show the chemical reactions which occur when chlorine is added to water. 

Chlorine Demand

When chlorine enters water, it immediately begins to react with compounds found in the water.  The chlorine will react with organic compounds and form trihalomethanes.  It will also react with reducing agents such as hydrogen sulfide, ferrous ions, manganous ions, and nitrite ions.  

Let's consider one example, in which chlorine reacts with hydrogen sulfide in water.  Two different reactions can occur:

Hydrogen Sulfide + Chlorine + Oxygen Ion Elemental  Sulfur + Water + Chloride Ions

H₂S + Cl₂ + O^2-  S + H₂O + 2Cl^- 

Hydrogen Sulfide + Chlorine + Water  Sulfuric Acid + Hydrochloric Acid

H₂S + 4Cl₂ + 4 H₂O  H₂SO₄ + 8 HCl

I have written each reaction using both the chemical formula and the English name of each compound.  In the first reaction, hydrogen sulfide reacts with chlorine and oxygen to create elemental sulfur, water, and chloride ions.  The elemental sulfur precipitates out of the water and can cause odor problems.  In the second reaction, hydrogen sulfide reactions with chlorine and water to create sulfuric acid and hydrochloric acid. 

Each of these reactions uses up the chlorine in the water, producing chloride ions or hydrochloric acid which have no disinfecting properties.  The total amount of chlorine which is used up in reactions with compounds in the water is known as the chlorine demand.  A sufficient quantity of chlorine must be added to the water so that, after the chlorine demand is met, there is still some chlorine left to kill microorganisms in the water.

Reactions of Chlorine Gas With Water

At the same time that chlorine is being used up by compounds in the water, some of the chlorine reacts with the water itself.  The reaction depends on what type of chlorine is added to the water as well as on the the pH of the water itself.

Chlorine may be added as to water in the form of chlorine gas, hypochlorite, or chlorine dioxide.  All types of chlorine will kill bacteria and some viruses, but only chlorine dioxide will effectively kill Cryptosporidium, Giardia, protozoans, and some viruses.  We will first consider chlorine gas, which is the most pure form of chlorine, consisting of two chlorine atoms bound together. 

Chlorine gas is compressed into a liquid and stored in metal cylinders.  The gas is difficult to handle since it is toxic, heavy, corrosive, and an irritant.  At high concentrations, chlorine gas can even be fatal.  

When chlorine gas enters the water, the following reaction occurs:

Chlorine + Water  Hypochlorous Acid + Hydrochloric Acid

Cl₂ + H₂O  HOCl + HCl

The chlorine reacts with water and breaks down into hypochlorous acid and hydrochloric acid.  Hypochlorous acid may further break down, depending on pH:

Hypochlorous Acid ↔ Hydrogen Ion + Hypochlorite Ion

HOCl ↔ H⁺ + OCl^-

Note the double-sided arrows which mean that the reaction is reversible.  Hypochlorous acid may break down into a hydrogen ion and a hypochlorite ion, or a hydrogen ion and a hypochlorite ion may join together to form hypochlorous acid. 

The concentration of hypochlorous acid and hypochlorite ions in chlorinated water will depend on the water's pH.  A higher pH facilitates the formation of more hypochlorite ions and results in less hypochlorous acid in the water.  This is an important reaction to understand because hypochlorous acid is the most effective form of free chlorine residual, meaning that it is chlorine available to kill microorganisms in the water.  Hypochlorite ions are much less efficient disinfectants.  So disinfection is more efficient at a low pH (with large quantities of hypochlorous acid in the water) than at a high pH (with large quantities of hypochlorite ions in the water.) 

Hypochlorites

Instead of using chlorine gas, some plants apply chlorine to water as a hypochlorite, also known as a bleach.  Hypochlorites are less pure than chlorine gas, which means that they are also less dangerous.  However, they have the major disadvantage that they decompose in strength over time while in storage.  Temperature, light, and physical energy can all break down hypochlorites before they are able to react with pathogens in water. 

There are three types of hypochlorites - sodium hypochlorite, calcium hypochlorite, and commercial bleach:

• Sodium hypochlorite (NaOCl) comes in a liquid form which contains up to 12% chlorine.  
• Calcium hypochlorite (Ca(OCl)²), also known as HTH, is a solid which is mixed with water to form a hypochlorite solution.  Calcium hypochlorite is 65-70% concentrated.   
• Commercial bleach is the bleach which you buy in a grocery store.  The concentration of commercial bleach varies depending on the brand - Chlorox bleach is 5% chlorine while some other brands are 3.5% concentrated.

Hypochlorites and bleaches work in the same general manner as chlorine gas.  They react with water and form the disinfectant hypochlorous acid.  The reactions of sodium hypochlorite and calcium hypochlorite with water are shown below:

Calcium hypochlorite  + Water  Hypochlorous Acid + Calcium Hydroxide

Ca(OCl)₂ + 2 H₂O  2 HOCl + Ca(OH)₂

Sodium hypochlorite + Water  Hypochlorous Acid + Sodium Hydroxide

NaOCl + H₂O  HOCl + NaOH

In general, disinfection using chlorine gas and hypochlorites occurs in the same manner.  The differences lie in how the chlorine is fed into the water and on handling and storage of the chlorine compounds.  In addition, the amount of each type of chlorine added to water will vary since each compound has a different concentration of chlorine. 

Chloramines

Some plants use chloramines rather than hypochlorous acid to disinfect the water.  To produce chloramines, first chlorine gas or hypochlorite is added to the water to produce hypochlorous acid.  Then ammonia is added to the water to react with the hypochlorous acid and produce a chloramine.

Three types of chloramines can be formed in water - monochloramine, dichloramine, and trichloramine.  Monochloramine is formed from the reaction of hypochlorous acid with ammonia:

Ammonia + Hypochlorous Acid  Monochloramine + Water

NH₃ + HOCl  NH₂Cl + H₂O

Monochloramine may then react with more hypochlorous acid to form a dichloramine:

Monochloramine + Hypochlorous Acid  Dichloramine + Water

NH₂Cl + HOCl  NHCl₂ + H₂O

Finally, the dichloramine may react with hypochlorous acid to form a trichloramine:

Dichloramine + Hypochlorous Acid  Trichloramine + Water

NHCl₂ + HOCl  NCl₃ + H₂O

The number of these reactions which will take place in any given situation depends on the pH of the water.  In most cases, both monochloramines and dichloramines are formed.  Monochloramines and dichloramines can both be used as a disinfecting agent, called a combined chlorine residual because the chlorine is combined with nitrogen.  This is in contrast to the free chlorine residual of hypochlorous acid which is used in other types of chlorination.

Chloramines are weaker than chlorine, but are more stable, so they are often used as the disinfectant in the distribution lines of water treatment systems.  Despite their stability, chloramines can be broken down by bacteria, heat, and light.  Chloramines are effective at killing bacteria and will also kill some protozoans, but they are very ineffective at killing viruses.

Breakpoint Chlorination

The graph below shows what happens when chlorine (either chlorine gas or a hypochlorite) is added to water.  First (between points 1 and 2), the water reacts with reducing compounds in the water, such as hydrogen sulfide.  These compounds use up the chlorine, producing no chlorine residual.  

Next, between points 2 and 3, the chlorine reacts with organics and ammonia naturally found in the water.  Some combined chlorine residual is formed - chloramines.  Note that if chloramines were to be used as the disinfecting agent, more ammonia would be added to the water to react with the chlorine.  The process would be stopped at point 3.  Using chloramine as the disinfecting agent results in little trihalomethane production but causes taste and odor problems since chloramines typically give a "swimming pool" odor to water.

In contrast, if hypochlorous acid is to be used as the chlorine residual, then chlorine will be added past point 3.  Between points 3 and 4, the chlorine will break down most of the chloramines in the water, actually lowering the chlorine residual.

Finally, the water reaches the breakpoint, shown at point 4.  The breakpoint is the point at which the chlorine demand has been totally satisfied - the chlorine has reacted with all reducing agents, organics, and ammonia in the water.  When more chlorine is added past the breakpoint, the chlorine reacts with water and forms hypochlorous acid in direct proportion to the amount of chlorine added.  This process, known as breakpoint chlorination, is the most common form of chlorination, in which enough chlorine is added to the water to bring it past the breakpoint and to create some free chlorine residual. 

Chlorine Dioxide

There is one other form of chlorine which can be used for disinfection - chlorine dioxide.  We have not discussed chlorine dioxide previously because it disinfects using neither hypochlorous acid nor chloramines and is not part of the breakpoint chlorination process.

Chlorine dioxide, ClO₂, is a very effective form of chlorination since it will kill protozoans,  Cryptosporidium, Giardia, and viruses that other systems may not kill.  In addition, chlorine dioxide oxidizes all metals and organic matter, converting the organic matter to carbon dioxide and water.  Chlorine dioxide can be used to remove sulfide compounds and phenolic tastes and odors.  When chlorine dioxide is used, trihalomethanes are not formed and the chlorination process is unaffected by ammonia.  Finally, chlorine dioxide is effective at a higher pH than other forms of chlorination.

So why isn't chlorine dioxide used in all systems?  Chlorine dioxide must be generated on site, which is a very costly process requiring a great deal of technical expertise.  Unlike chlorine gas, chlorine dioxide is highly combustible and care must be taken when handling the chlorine dioxide.

Part 3: Efficiency and Equipment

Efficiency
Residual and Dosage

A variety of factors can influence disinfection efficiency when  using breakpoint chlorination or chloramines.  One of the most important of these is the concentration of chlorine residual in the water.

The chlorine residual in the clearwell should be at least 0.5 mg/L.  This residual, consisting of hypochlorous acid and/or chloramines, must kill microorganisms already present in the water and must also kill any pathogens which may enter the distribution system through cross-connections or leakage.   In order to ensure that the water is free of microorganisms when it reaches the customer, the chlorine residual should be about 0.2 mg/L at the extreme ends of the distribution system.  This residual in the distribution system will also act to control microorganisms in the distribution system which produce slimes, tastes, or odors. 

Determining the correct dosage of chlorine to add to water will depend on the quantity and type of substances in the water creating a chlorine demand.  The chlorine dose is calculated as follows:

Chlorine Dose = Chlorine Demand + Chlorine Residual

So, if the required chlorine residual is 0.5 mg/L and the chlorine demand is known to be 2 mg/L, then 2.5 mg/L of chlorine will have to be added to treat the water. 

The chlorine demand will typically vary over time as the characteristics of the water change.  By testing the chlorine residual, the operator can determine whether a sufficient dose of chlorine is being added to treat the water.  In a large system, chlorine must be sampled every two hours at the plant and at various points in the distribution system. 

It is also important to understand the breakpoint curve when changing chlorine dosages.  If the water smells strongly of chlorine, it may not mean that too much chlorine is being added.  More likely, chloramines are being produced, and more chlorine needs to be added to pass the breakpoint. 

Contact Time

Contact time is just as important as the chlorine residual in determining the efficiency of chlorination.  Contact time is the amount of time which the chlorine has to react with the microorganisms in the water, which will equal the time between the moment when chlorine is added to the water and the moment when that water is used by the customer.  The longer the contact time, the more efficient the disinfection process is.  When using chlorine for disinfection a minimum contact time of 30 minutes is required for adequate disinfection. 

The CT value is used as a measurement of the degree of pathogen inactivation due to chlorination.  The CT value is calculated as follows:

CT = (Chlorine residual, mg/L) (Contact time, minutes)

The CT is the Concentration multiplied by the Time.  As the formula suggests, a reduced chlorine residual can still provide adequate kill of microorganisms if a long contact time is provided.  Conversely, a smaller chlorine residual can be used as long as the chlorine has a longer contact time to kill the pathogens. 

Other Influencing Factors

Within the disinfection process, efficiency is influenced by the chlorine residual, the type of chemical used for chlorination, the contact time, the initial mixing of chlorine into the water, and the location of chlorination within the treatment process.  The most efficient process will have a high chlorine residual, a long contact time, and thorough mixing. 

Characteristics of the water will also affect efficiency of chlorination.  As you will recall, at a high pH, the hypochlorous acid becomes dissociated into the ineffective hypochlorite ion.  So lower pH values result in more efficient disinfection. 

Temperature influences chlorination just as it does any other chemical reaction.  Warmer water can be treated more efficiently since the reactions occur more quickly.  At a lower water temperature, longer contact times or higher concentrations of chemicals must be used to ensure adequate disinfection. 

Turbidity of the water influences disinfection primarily through influencing the chlorine demand.  Turbid water tends to contain particles which react with chlorine, reducing the concentration of chlorine residual which is formed.  Since the turbidity of the water depends to a large extent on upstream processes (coagulation, flocculation, sedimentation, and filtration), changes in these upstream processes will influence the efficiency of chlorination.  Turbidity is also influenced by the source water - groundwater turbidity tends to change slowly or not at all while the chlorine demand of surface water can change continuously, especially during storms and the snow melt season.

Finally, and most intuitively, the number and type of microorganisms in the water will influence chlorination efficiency.  Since cyst-forming microorganisms and viruses are very difficult to kill using chlorination, the disinfection process will be less efficient if these pathogens are found in the water. 

 Choosing a Disinfection Method

Of the many disinfection methods, five have been used extensively in water treatment.  The table below lists some of the factors which may influence the choice of treatment method in a new plant.

	Chlorine (Gas or Hypochlorite)	Chlorine Dioxide	Chloramine	Ozone	Ultraviolet
Produces trihalomethanes?	yes	no	yes	sometimes	no
Produces other troublesome byproducts?	yes	yes	yes	yes	sometimes
Impacted by lime softening?	yes	no	yes	no	yes
Impacted by turbidity?	somewhat	somewhat	somewhat	somewhat	yes
Meets Giardia removal standards?	no	yes	no	yes	no
Meets Cryptosporidium removal standards?	no	no	no	yes	no
Meets virus removal standards?	yes	yes	no	yes	yes
Operator skill level	low	high	low/medium	high	medium
Applicable to large utilities?	yes	yes	yes	yes	no
Applicable to small utilities?	yes	yes	yes	yes	yes

You may note that many of the disinfection methods do not meet standards for Giardia, Cryptosporidium, and virus removal.  This does not mean that these disinfection methods cannot be used.  When used in conjunction with filtration, all of the disinfection methods can be used to meet removal standards. 

Wasserprojekt in Moskau

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S.550

11.8. Redox-Spannung